Facsimile transmission of documents currently is accomplished by means of optical scanners, receivers and printers which have a relatively high black and white resolution of typically 80 to 120 dots per centimeter (200 to 300 dots per inch). By contrast, the resolution of video monitors and other display peripherals which are ordinarily used in connection with business and personal computers range from 25 to 50 dots per centimeters (60 to 120 dots per inch). Accordingly, there are no practical ways to legibly and conveniently receive and display facsimile transmissions on such low resolution systems, except by displaying only a fraction of the facsimile document at a time on the video screen. A facsimile transmission or similarly digitized document can be viewed as a large array (typically 1,728 X 1,075) of elements of information, also called "pels", either black or white. To represent such a document in its entirety on a single screen memory of a personal computer requires scaling down this array into a smaller array (typically 640 X 819) of pels.
Such an operation has received some attention in the past. Reduction methods range from simply discarding preselected rows and columns of pels through combining clusters of pels with or without reference to the values of neighboring pels. The former is disclosed in U.S. Pat. No. 4,280,143 Judd, and the latter in U.S. Pat. No. 3,976,982 Eiselen. Another approach, called "fast projection method" proposed by H. P. Morita et al. in "A Resolution Conversion Scheme for Black and White Images, IEEE Global Telecommunications Conference Record", San Diego, Calif., 1983, pages 1255-1260, takes a weighted average of neighboring pels to assign values to pels in a reduced image. In IBM Technical Disclosure Bulletin, Volume 27, Number 5 Oct. 1984, pages 2984-2986 M. Kikutani proposes a font-scaling method which makes use of geometric reference points and lines in order to scale characters.
Scale-down methods which discard whole pels degrade the readability of the resulting display to an impractical level. Simply combining clusters of pels with a logical OR operation, though very fast and practical, does not yield a much more legible display than discarding whole pels, since no consideration is given in this method to pels which fall in the same cluster as one black pel. More elaborate methods of "cluster combining" fail to achieve the simplicity necessary for speed and cost-efficiency. The best trade-off between speed and quality is described in Eiselen, supra, as a simple counting process of the black pels within a cluster, followed by a thresholding fixed at half the total number of pels within the cluster. However, all prior art methods transform black and white pels into black or white pels. The constraint imposed on them by the binary nature of the output is at the source of either the lack of quality resulting from the discarding of information, or the slowness of the process in view of complex information preserving procedures.
It is also a common feature of all methods described in the prior art, either explictly or implicitly, that the input array is first made available in an incompressed format such as a bit map, which can be directly addressed along its two dimensions, or bit stream, in which each input pel receives a distinct physical representation. However, the most common method of facsimile transmission such as the CCITT Group III handles the digitized document as a compressed signal constituted by a sequence of either black or white runs.
It would be advantageous to have a rapid and efficient apparatus or method to give business and personal computer stations the ability to process facsimile transmissions in their compressed signal form and display them legibly as a whole on a video monitor screen.